1. Field of the Invention
The present invention relates to a method of, and an apparatus for, synthesizing a sound from coded parameters sent from a transmitter, and also to a method of, and an apparatus for, expanding the band of a narrow frequency-band sound or speech signal transmitted to a receiver from the transmitter over a communications network such as a telephone line or broadcasting network, while keeping the frequency band unchanged over the transmission path.
2. Description of Related Art
The telephone lines are regulated to use a frequency band as narrow as 300 to 3,400 Hz, for example, and the frequency band of a sound signal transmitted over the telephone network is thus limited. Therefore, the conventional analog telephone line may not be said to assure a good sound quality. This is also true for the digital portable telephone.
However, since the standards, regulations and rules for the telephone transmission path are already strictly defined, it is difficult to expand the frequency band for such specific communications. In these situations, there have been proposed various approaches to generate a wide-band signal by predicting out-of-band signal components at the receiver. Among such technical proposals, an approach to overcome such a difficulty by using a sound code book mapping is considered the best for a good sound quality. This approach is characterized by that two sound code books for sound analysis and synthesis are used to predict a spectrum envelope of a wide-band sound from a one of a narrow-band sound supplied to the receiver.
More particularly, the above approach uses the Linear Predictive Code (LPC) cepstrum, a well-known parameter for representation of a spectrum envelope, to pre-form two sound code books, one for a narrow-band sound and the other for a wide-band sound. There exist one-to-one correspondences between code vectors in these two sound code books. A narrow-band LPC cepstrum is determined from an input narrow-band sound, quantized in vector by comparison with a code vector in the narrow-band sound code book, and dequantized using a corresponding code vector in the wide-band sound code book, to thereby determine a wide-band LPC cepstrum.
For the one-to-one correspondence between the code vectors, the two sound code books are generated as will be described below. First, a wide-band learning sound is prepared, and it is limited in bandwidth to provide a narrow-band learning sound as well. The wide- and narrow-band learning sounds thus prepared are framed, respectively, and an LPC cepstrum determined from the narrow-band sound is used to first learn and generate a narrow-band sound code book. Then, frames of a learning wide-band sound corresponding to the resultant learning narrow-band sound frames to be quantized to a code vector are collected, and weighted to provide wide-band code vectors from which a wide-band sound code book is formed.
As another application of this approach, a wide-band sound code book may first be generated from the learning wide-band sound, and then corresponding learning narrow-band sound frames are weighted to provide narrow-band code vectors from which a narrow-band sound code book is generated.
Further, there has also been proposed a sound code book generation mode in which an autocorrelation is used as a parameter to be a code vector. Also, innovations are requisite for the LPC analysis and synthesis. Such innovations include a set of an impulse train and noise, an upsampled narrow-band innovation, etc.
The application of the aforementioned approaches have not succeeded in attaining a satisfactory sound quality. In particular, the sound quality is remarkably poor when the approach is applied for a sound encoded in the low bit rate sound encoding mode such as the Vector Sum Excited Linear Prediction (VSELP) mode, Pitch Synchronous Innovation-Code Excited Linear Prediction (PSI-CELP) mode or the like included in the so-called sound encoding mode CELP (Code Excited Linear Prediction) adopted in the digital telephone systems currently prevailing in Japan.
Also, the size of the memory used in generating the narrow- and wide-band sound code books is insufficient.